feat(chat): demo-chat seed for the screenshot sweep (debug)

Add App::seedChatDemoData() + a "Seed demo chat" button in the DEBUG OPTIONS
card (shown only in chat-enabled builds, next to the screenshot sweep). It gives
the Chat tab a demo identity and injects a few sample conversations into the
in-memory store (NOT persisted — no DB attached, gone on restart) so the
screenshot sweep captures the tab's real UI (conversation list, thread bubbles,
composer, and the "waiting for reply" contact-request state) instead of the
empty "unlock" hint. No-op when the chat feature is off.

Verified: Linux + Windows build with chat ON, ctest 100%, hygiene clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app.h

@@ -174,6 +174,10 @@ public:
     // message (to a conversation whose peer key we know) / a new-conversation contact request.
     void sendChatMessage(const std::string& conversationId, const std::string& text);
     void startChatConversation(const std::string& peerZaddr, const std::string& text);
+    // Debug/sweep convenience: give the Chat tab a demo identity + a few sample conversations
+    // (in-memory only, not persisted) so the screenshot sweep captures the populated UI. No-op when
+    // the chat feature is off.
+    void seedChatDemoData();
     // Reason the lite wallet failed to auto-open this session (empty if none / opened OK).
     const std::string& liteOpenError() const { return lite_open_error_; }
     // Show the lite send-time unlock modal (called when a spend is attempted on a locked wallet).